1. Technical Field
The present invention generally relates to a refrigerant compressor, and more particularly, to a slant plate type compressor, such as a wobble plate type compressor, with a variable displacement mechanism which is suitable for use in an automotive air conditioning system.
2. Description of The Prior Art
Slant plant type piston compressors including variable displacement or capacity adjusting mechanisms for controlling the compression ratio of a compressor in response to demand are generally known in the art. For example, Japanese Utility Model Application Publication No. 63-134181 discloses a wobble plate type compressor including a cam rotor driving device and a wobble plate linked to a plurality of pistons. Rotation of the cam rotor driving device causes the wobble plate to nutate and thereby successively reciprocate the pistons in the corresponding cylinders. The stroke length of the pistons and thus the capacity of the compressor may be easily changed by adjusting the slant angle of the wobble plate. The slant angle is changed in response to the pressure differential between the crank chamber and the suction chamber.
In the above-mentioned Japanese Utility Model Application Publication, the crank chamber and the suction chamber are linked in fluid communication by a first path or passageway. A valve mechanism is disposed in the first passageway in order to control fluid communication between the crank and suction chambers by the opening and closing of the first passageway. The valve mechanism generally includes a pressure sensing device for sensing pressure in the suction chamber, a solenoid, a plunger and a valve member fixedly connected to both the pressure sensing device and one end of the plunger. The solenoid receives two external signals, one of which represents the heat load on an evaporator of a cooling circuit and another of which represents the amount of demand for accelerating an automobile.
The valve member opens and closes the first passageway in response to changes in the suction chamber pressure so as to change the crank chamber pressure relative to the suction chamber pressure. This then results in a change in the angular position of the wobble plate so that the capacity displacement of the compressor is adjusted and a control point of the suction chamber pressure is maintained at a predetermined constant value.
The solenoid induces various electromagnetic attraction forces responsive to changes in the two external signals to thereby change the axial position of the plunger. This then changes the control point of the suction chamber pressure from predetermined maximum to predetermined minimum values.
The compressor further includes a second passageway, separate from the first passageway, which communicates between the crank chamber and the suction chamber. A safety valve device, including a ball member and a coil spring elastically supporting the ball member, is disposed in the second passageway. The safety valve device opens and closes the second passageway in response to changes in the pressure differential between the crank chamber and the suction chamber. The second passageway is opened when the pressure differential between the crank chamber and the suction chamber exceeds a predetermined value. Therefore, when communication between the crank chamber and the suction chamber is blocked for a long time period due to trouble in the valve mechanism, thereby causing an abnormal rise in the crank chamber pressure because of blow-by gas leaking past the pistons in the cylinders as the pistons reciprocate, the second passageway is opened to forcibly and quickly reduce the crank chamber pressure and thereby prevent an abnormal pressure differential between the crank and suction chambers. As a result, excessive friction between internal component parts of the compressor caused by abnormal pressure differential between the crank chamber and the suction chamber can be prevented.
In this prior art embodiment, however, since the second passageway is separate from the first passageway, the process of forming the second passageway and the process of disposing the safety valve device in the second passageway are additional steps required during the manufacturing of the compressor. Accordingly, the manufacturing process for the compressor is complicated. Therefore, it is highly desirable to provide a compressor having a variable displacement control mechanism which can be easily manufactured and which can prevent an abnormal pressure differential between the crank and suction chambers.